Due to the explosive proliferation of the portable terminal market in recent years and improvements in the infrastructure associated therewith, stricter requirements have been made from the market for improvements in the efficiency of transmission amplifiers for base stations.
In order to respond to the foregoing requirements, attention has been focused in recent years on the trend of attempts to build high-performance and highly efficient amplifiers by combining technology to amplify signals at high efficiency, as represented by the Doherty amplifier, with technology to reduce distortions therefor together with recent distortion compensation technology.
The Doherty amplifier is a device for improving the efficiency of a high-output power amplifier, which was first proposed in Document 1 (W. H. Doherty “A New High Efficiency Power Amplifier for Modulated Waves”, Proc.
IRE, Vol. 24, No. 9, September in 1936).
The Doherty amplifier comprises a carrier amplifier for performing an amplifying operation at all times; and a peak amplifier for performing an amplifying operation when high power is generated, specifically, only after the carrier amplifier has reached a saturated maximum output.
In the Doherty amplifier, devices having the same characteristics are generally used for the carrier amplifier and peak amplifier, and they are arranged in parallel. A large number of Doherty amplifiers have been actually implemented as amplifiers to handle signals in frequency bands from low frequencies to millimeter waves.
The example described in Document 2 (JP-7-22852-A) is an example of such a kind of Doherty amplifiers that is conventionally used. FIG. 1 illustrates the Doherty amplifier described in Document 2. In the following, the Doherty amplifier described in Document 2 will be described in brief with reference to FIG. 1.
In FIG. 1, a signal applied from input terminal 1 is distributed to a side of the carrier amplifier and to a side of the peak amplifier by input branching circuit 2, which includes one-quarter wavelength transmission path 21. Carrier amplifier 3 amplifies a signal that is distributed to the carrier amplifier side. A signal distributed to the peak amplifier side is amplified by peak amplifier 4 after it has passed through one-quarter wavelength transmission path 21f. 
Output combiner circuit 5 includes one-quarter wavelength transmission path 51. Output combiner circuit 5 combines the output of carrier amplifier 3, which has passed one-quarter wavelength transmission path 51, with the output of peak amplifier 4 to deliver the resulting output. Therefore, a phase relationship between output signals of carrier amplifier 3 and peak amplifier 4 is identical at the signal combining point of output combiner circuit 5.
However, if an amplifying operation of carrier amplifier 3 or peak amplifier 4 of the Doherty amplifier differs from an ideal operation, the signal combination, which is performed by output combiner circuit 5, is not performed in an effective manner. For this reason, the Doherty amplifier fails to provide an ideal linear amplifying action and saturated output power.
For example, the foregoing problem occurs when devices having equivalent characteristics (for example, the gm-Id characteristic) are used for a carrier amplifier and a peak amplifier, which make up the Doherty amplifier (classical Doherty). In this event, a problem arises, particularly, in that the operation of the peak amplifier differs from optimum performance. Specifically, a problem occurs in that the gain in the peak amplifier is smaller than a optimum gain.
Therefore, an ideal linear amplifying action or saturated output power cannot be provided even if the carrier amplifier and peak amplifier are identical in the gm (transfer conductance) characteristic (FET's or the like).
Several approaches for addressing this problem have been proposed.
For example, Document 3 (RF Power Amplifiers for Wireless communications, written by Steve C. Cripps, p236, Artech House, 1999) has proposed a technique for controlling the attenuation amount of a variable attenuator, which is disposed on the input side of a peak amplifier, in accordance with the magnitude of the input level in order to compensate for the transfer characteristic.
Also, Document 4 (Advanced Techniques in RF Power Amplifiers written by Steve C. Cripps, P50, Artech House, 2002) has proposed a method of generating maximum power of a Doherty amplifier by appropriately controlling the bias setting of a carrier amplifier from a class-C bias to a class-B bias, in accordance with an input signal level, though no specific block diagram or the like is found therein.
Further, Document 5 (Published Japanese Translation of PCT International Publication for Patent Application No. 2000-513535) has proposed techniques by which a detector directly or indirectly detects the power level of an input signal and the magnitude of the signal, such that bias controllers of a carrier amplifier and a bias amplifier control biases for the carrier amplifier and peak amplifier, respectively, relying on the detected value.
However, whether it be the techniques of Documents 3, 4 or 5, they all requires circuits for making determinations, control and the like, thus leading to a problem that the configuration becomes complicated.